1. Field of the Invention
The invention relates to an agitator mill comprising a grinding receptacle, which has a cylindrical wall, a bottom, and a cover; a grinding chamber formed in the grinding receptacle; an agitator unit, which has a driving shaft, which is disposed substantially outside the grinding chamber and driven in rotation, and a rotor, which is located in the grinding chamber and connected to the driving shaft; agitator elements mounted on the rotor, which are disposed in radial planes radially of the central longitudinal axis, and which project into the grinding chamber radially of the central longitudinal axis towards the wall of the grinding receptacle; counterpart elements mounted on the wall of the grinding receptacle, which are disposed in radial planes radially of the central longitudinal axis, and which project towards the rotor into the grinding chamber; a grinding-stock inlet, which leads through the bottom into the grinding chamber; an auxiliary-grinding-body separating device disposed in the vicinity of the cover and which is disposed upstream of a grinding-stock outlet; and a filling of auxiliary grinding bodies, with a diameter a, in the grinding chamber.
2. Background Art
Agitator mills of the generic type pose the fundamental problem that the auxiliary grinding bodies are entrained in the grinding-stock flow direction and accumulate in front of the separating device, which may lead to impairment of the grinding and dispersing process and, in the extreme, to the agitator mill being blocked.
For avoidance of the mentioned effect, U.S. Pat. No. 4,848,676 teaches, in the case of an agitator mill of the generic type, to provide a device for detection of the distribution of the auxiliary grinding bodies in the grinding chamber, which comprises a measuring point for detection of the pressure drop in the grinding chamber as compared to atmospheric, this measuring point sensing the grinding-stock pressure directly before the grinding chamber, with transgression of a given pressure drop constituting a measure for a concentration of auxiliary grinding bodies at the grinding-stock inlet and in front of the separating device, respectively. Furthermore, a device is provided for the regular distribution of the auxiliary grinding bodies in the grinding chamber, the effect of which resides in a reduction of the mass flow when the auxiliary grinding bodies concentrate in front of the separating device. This design has proved very successful, but it requires some implementation in terms of measuring and regulation.
DE 32 45 825 A1 teaches to provide a device in an agitator mill, which selectively exercises, at least substantially only on the auxiliary grinding bodies, a force in the opposite direction of the flow of grinding stock. This is to prevent the auxiliary grinding bodies from migrating before the separating device. For detection of auxiliary grinding bodies that arrive before the separating device, a pressure sensor is provided, by means of which the pressure of the auxiliary grinding bodies is sensed in this area.
It is an object of the invention to embody an agitator mill of the generic type in such a way that accumulation of the auxiliary grinding bodies before the separating device is at least largely precluded by simple means.
According to the invention, this object is attained by the features which consist in that the distance from each other of agitator elements and adjacent counterpart elements in the direction of the central longitudinal axis decreases from a greatest distance c in the vicinity of the grinding-stock inlet to a smallest distance b in the vicinity of the auxiliary-grinding-body separating device. Reducing the distance of agitator elements from neighboring counterpart elements towards the separating device helps obtain a higher local shear intensity and thus an increased power density as the distance decreases. As the auxiliary grinding bodies tend to avoid the status of increased power density, they migrate to the area of lower power density, i.e. to the area where the agitator elements have greater distances from the neighboring counterpart elements. This effect is of special importance when a very tough i.e., very viscous, grinding stock is ground. The grinding i.e., the comminution, of the grinding stock particles, increases the surface thereofxe2x80x94related to their mass. This increases the toughness of the grinding stock in the flow direction. The growing toughness leads to increasingly higher entrainment forces in the flow direction which would give rise to an accumulation of auxiliary grinding bodies before the separating device. This effect is opposed by the described measures. The comminution effect intensifies as the grinding process progresses during the flow of the grinding stock through the grinding chamber.
Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.